Conventionally, as a line transition device for coupling different types of transmission lines, there is known a line transition device formed by inserting part of a planar circuit (microstrip line) provided on a dielectric substrate into a waveguide in a conductor block. Examples of such a line transition device are disclosed in Patent Document 1 and Patent Document 2.
FIG. 1(A) illustrates an exemplary configuration of a line transition device described in Patent Document 1. A line transition device 1 is formed by providing grooves 4A and 4B constituting a waveguide 4 in respective conductor blocks 2 and 3, which are separated by a plane parallel to the E-plane of the waveguide, and inserting part of a dielectric substrate 5 into the waveguide 4 in a direction parallel to the E-plane. The dielectric substrate 5 is provided with a line conductor 6 and a ground conductor 7 of a microstrip line. Ends of the line conductor 6 and the ground conductor 7 are positioned at the terminal end of the waveguide 4. In the waveguide 4, the line conductor 6 and the ground conductor 7 are close to the H-plane of the waveguide 4 and each have a plurality of open stubs (not shown) having a stub length equal to a quarter of the wavelength of electromagnetic waves. Through the open stubs, conductors of the waveguide 4 are coupled to the line conductor 6 and the ground conductor 7 at high frequencies.
In such a line transition device, if a gap is created at the interface between a conductor block having a waveguide and a dielectric substrate having transmission lines, spurious electromagnetic waves may be generated in the gap and cause an increase in radiation loss.
Patent Document 2 proposes a configuration illustrated in FIG. 1(B) as a solution to this problem. As in the case of the configuration described above, a line transition device 1 of FIG. 1(B) has a waveguide 4 in a conductor block 2. To solve the problem described above, the line transition device 1 of FIG. 1(B) is provided with a choke groove G22 surrounding the terminal end of the waveguide 4. Since this suppresses generation of spurious electromagnetic waves in a gap at the interface between the conductor block 2 and a dielectric substrate (not shown), a line transition device with less radiation loss can be provided.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 5-335815    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2004-147291
Although the line transition device disclosed in Patent Document 1 allows good coupling of the ground and line conductors to conductors of the waveguide, it is not directed to the suppression of spurious electromagnetic waves in a gap between the dielectric substrate and the conductor block. Moreover, the line transition device disclosed in Patent Document 1, where coupling to the waveguide is made through a plurality of open stubs, requires extremely fine electrodes to deal with high frequency waves (millimeter waves and microwaves) in the microstrip line. This not only makes microfabrication difficult, but may cause interdigital electrodes to break or float and degrade the reliability of the stubs.
On the other hand, to effectively block spurious electromagnetic waves, the line transition device disclosed in Patent Document 2 requires, for example, a square U-shaped choke groove substantially entirely surrounding the terminal end of the waveguide and thus requires a conductor block of large size.
For compactness, a choke groove that only partially surrounds the terminal end of the waveguide may be provided. However, this causes a problem in that spurious electromagnetic waves cannot be sufficiently suppressed. Moreover, since spurious electromagnetic waves cause equivalent short-circuit points of the waveguide to be displaced from each other, the coupling between the waveguide and a planar circuit is weakened.